1. Field Of the Invention
This invention relates to an optical wavelength converter element for converting a fundamental wave into a second harmonic wave, or the like, and, more particularly, to an optical wavelength converter element having periodic domain reversals.
Moreover, this invention relates to a method for creating domain reversals in a predetermined pattern on a ferroelectric substance having a non-linear optical effect in order to fabricate the foregoing optical wavelength converter element.
2. Description of the Prior Art
A proposal has already been made by Bleombergen et al. in Physics Review vol. 127, No. 6 in 1918 (1962), in which the wavelength of a fundamental wave is converted into a second harmonic wave using an optical wavelength converter element with regions (domains) where the spontaneous polarization of a ferroelectric substance possessing a nonlinear optical effect is periodically inverted.
In this method, the fundamental wave can be phase matched with the second harmonic wave by setting the period .LAMBDA. of the domain reversals to be an integral multiple of a coherence length .LAMBDA.c which is given by EQU .LAMBDA.c=2.pi./{.beta.(2.omega.)-2.beta.(.omega.)} (1)
where .beta.(2.omega.) designates the propagation constant of the second harmonic wave, and 2.beta. (.omega.) represents the propagation constant of the fundamental wave. When wavelength conversion is effected using a bulk crystal made of a nonlinear optical material, a wavelength to be phase-matched is limited to a specific wavelength inherent to the crystal. However, in accordance with the above described method, phase matching can be efficiently carried out by selecting a period .LAMBDA. which satisfies the condition (1) for an arbitrary wavelength.
One example of the known fabrication methods for such periodic domain reversals is the method proposed by H. Ito, C. Takyu, and H. Inaba in Electronics Letters, vol. 27, No. 14, pp. 1221 (1991), wherein the -z plane of LiTaO.sub.3 or LiNbO.sub.3 is directly exposed to electron beams at room temperature. In this example, electron beams are irradiated to a ferroelectric substance while an earth electrode is formed on the opposite side (the +z plane in the above example) thereof relative to its exposed side.
In an optical wavelength converter element fabricated by the above fabrication method, periodic domain reversals are created throughout the thickness of an LiNbO.sub.3 substrate (e.g. around 0.5 mm in thickness). In other words, the periodic domain reversals are created to extend from the -z plane to the +z plane of the substrate. For this reason, this type of element can be applied a bulk type optical wavelength converter element. In a wavelength converter element in which third-order periodic domain reversals are created in accordance with the previous method, phase matching is observed in the bulk by the wavelength sweep of a Ti:Al.sub.2 O.sub.3 laser.
Also, as described in Japanese Unexamined Patent Publication No. 6(1994)-138506, it is possible to create periodic domain reversals by irradiating ion beams, which are an example of high energy beams, the same as electron beams, onto a ferroelectric substance instead of irradiating the electron beams.
However, in the existing method for creating domain reversals by the irradiation of high energy beams such as electron beams and ion beams, it is widely known that the geometry of the domain reversals is inferior in evenness and reproducibility. Moreover, in this existing method, it is difficult for the domain reversals to be formed in an ideal linear pattern. Hence, it is also known that the domain reversals become uneven in thickness or dot-shaped in most cases.